1. Field of the Invention
The invention relates generally to the fabrication of semiconductor devices. The invention relates more specifically to a method for forming a high quality buried isolation layer in a SIMOX structure.
2a. Cross Reference to Related Applications
The following copending U.S. patent application(s) is/are assigned to the assignee of the present application, is/are related to the present application and its/their disclosures is/are incorporated herein by reference:
(A) Ser. No. 07/861,141 filed Mar. 31, 1992, by Tatsuo Nakato and entitled, GRADED IMPLANTATION OF OXYGEN AND/OR NITROGEN CONSTITUENTS TO DEFINE BURIED ISOLATION REGION IN SEMICONDUCTOR DEVICES; the latter having been abandoned and continued by way of application Ser. No. 08/232,880, filed Apr. 25, 1994 which then issued as U.S. Pat. No. 5,395,771 on Mar. 7, 1995.
3. Description of the Related Art
SIMOX devices (Separation by IMplanted OXygen) employ an implanted layer of silicon oxide between an active device layer and a bulk portion of a supporting substrate. The implanted dielectric layer provides DC isolation between the substrate bulk portion and active devices in the active device portion of the substrate. The dielectric layer also minimizes capacitance between the active devices and the bulk portion of the substrate.
A typical SIMOX fabrication process includes implanting a layer of oxygen atoms at a dose of approximately 0.2.times.10.sup.18 atoms/cm.sup.2 -2.0.times.10.sup.18 atoms/cm.sup.2 with an energy of about 20-200 KeV in a temperature range of about 450.degree. C.-650.degree. C. This implant is followed by annealing at a temperature of about 1150.degree. C.-1400.degree. C. The annealing steps removes some of the implantation damage, distributes the implanted oxygen atoms among neighboring silicon atoms and helps the implanted oxygen atoms to react with the neighboring silicon atoms, thereby forming a buried, stoichiometric SiO.sub.2 layer.
Generally speaking, the SIMOX process is a rather expensive undertaking due to the large amount of oxygen implantation necessary and the lengthy amount of time required to complete implantation. If time and energy are to be invested in the formation of devices having implanted dielectric layers, it is desirable to have a cost-effective method for assuring high yields in mass production environments.
Several problems exist within the conventional SIMOX process. One of them is an undesirable formation of small pin holes or thinned sections in the dielectric layer. This occurs because unwanted contamination particles are often found at random spots on the substrate surface during oxygen implantation. Some of the surface contaminating particles completely block oxygen ions from entering the substrate. Others of the surface contaminating particles reduce the velocity of some of the oxygen ions that are to be implanted at a desired depth below the substrate surface and thereby divert them to different depth. Complete blockage produces a pin hole. Diversion due to velocity reduction produces a thin section. Sometimes a small contaminating particle completely blocks oxygen ions in a small area but the resulting pin-hole is partially filled by scattering from adjacent implant beams. This also produces a thin section.
Pin-holes and/or thin sections in a buried dielectric layer disadvantageously increase leakage current through the dielectric layer and/or decrease the breakdown voltage of the dielectric layer. This adversely affects the ultimate performance of devices formed under the SIMOX process and tends to reduce mass production yields.